1. Field of Invention
This invention relates to electronic circuits involving "totem pole" connected Field-Effect Transistors (FETs) or Insulated-Gate Bipolar Transistors (IGBTs).
2. Background Art
Connecting two FETs in series between two voltages allows the center connection to be switched to either supply voltage, provided that appropriate control voltages are connected to the gates of the two FETs. Such "totem pole" configurations of FETs are commonly used in high voltage and/or high current switching applications, such as in driving high voltage PIN diodes, controlling motors with "full-bridge" drivers, piezoelectric actuator drivers, and DC-DC power converters, among other uses.
Power FETs that are suited to the applications addressed herein are available as either N-channel or P-channel types. N-Channel devices are preferable to P-Channel types in terms of cost, breakdown voltage, and efficiency. but they require a control voltage which is more positive than the positive supply to be switched. This condition is sometimes not a problem in the case of the more negative of the two FETs in a totem pole pair, but usually the supply voltage connected to the positive-side FET is the most positive voltage available in the system.
In order to use an N-channel FET on the positive half of a totem-pole pair, then, frequently requires the generation of a voltage which is more positive than the highest voltage available in the system. Typically this voltage is generated by a DC-DC converter which has its negative output referenced to the positive supply voltage. In many cases it is necessary to generate a special voltage which is more positive, by a given amount, than the negative supply to be switched.
Additionally, totem pole FET configurations require a "dead time" during switching between the conduction phases of the two FETs to allow the off-going FET time to fully shut off. If an excessively high switching repetition rate is applied to the FET circuit, one or both FETs will be damaged if this "dead time" is not allowed for.
Integrated circuits are commercially available that incorporate capacitively coupled DC-DC converters to enable the use of FET totem pole architectures using only N-channel devices, but these IC's are only available for certain applications. Usually these IC's are configured as "high side" drivers, and they are therefore not useful when driving the negative side FET.
High-side voltages are usually generated by either capacitive or inductive means, and transformers are also sometimes used in this regard. Driver circuits are usually biased by these specially-generated voltages.
Opto-coupled FET drivers are available in IC form, and many designers create their own opto-coupled FET drivers by connecting an optoisolator to a FET driver. Optoisolation is used in these cases to allow freedom in level shifting of the control voltages to levels that are required by the FETs in a given application, as well as to eliminate feedback of potentially dangerous voltages spikes (which are generated by the switching of the FETs) to the control circuitry.
Currently available FET totem pole outputs typically switch directly to either one or the other voltage, and additional circuit complexity is required if it is desired to have either FET act as a "current source" or "current sink". A current source/sink style output is one in which the output current is made to be largely independent of the load voltage or it's dynamic characteristics.
In applications where FETs are used in high radiation environments, it is found that the Vgs(on) of the FETs will shift lower with increased exposure time, eventually to where a negative Vgs is required to shut off the FET completely. As a result of this, totem-pole FET circuits for use in these environments must be designed with provision for generating positive and negative Vgs control voltages, to provide enhanced robustness.
Some difficulties with the prior art are encountered when driving N-channel FETs in a totem pole configuration. These circuits tend to be fairly complex to perform each of the required functions correctly. Such circuits frequently must include "dead-time" and FET protection features which add to complexity. Many designs rely on optoisolators, which can be slow, large, and expensive. Protective isolation between the switching FETs and the control circuitry is sometimes necessary.
Many FET driver circuits require additional voltages to be created, and some of these circuits require large component. Typical FET driver circuits are not easily adapted to provide the positive and negative Vgs that is required in high radiation environments.